The present invention relates to method and apparatus for providing access between marine structures, and more especially for providing access from a vessel, such as a relatively small vessel, to a relatively larger vessel or to a fixed offshore structure, or between two vessels, especially larger vessels. In the case of transfer between vessels, the invention is particularly advantageous when the vessels are in open water and/or under way. Said access may be, or may Include, the transfer of personnel and/or the transfer of goods and equipment.
When at sea, or on similar large body of water such as larger lakes, it is often necessary for personnel or equipment to transfer from one “marine structure” to another. In the context of this application, the term “marine structure” refers to: each of two vessels of preferably of broadly similar size between which transfer is effected; or to each of a relatively small boat (“transfer vessel”) employed to carry personnel and equipment to a worksite and the worksite itself. The worksite might be a larger vessel, such as when a pilot needs to board the larger vessel from a pilot boat, or a fixed offshore structure such as an oil or gas rig, wind turbine or the like where maintenance workers need access from time to time. Most preferably one of the marine structures between which transfer is effected is a vessel navigable under its own power. In the context of transfer from a relatively small transfer vessel such as an RIB and given that the motion of a larger vessel is usually much less relative to the motion of a smaller transfer vessel, the term “fixed structure” will be used hereinafter to refer to both fixed offshore structures and relatively larger vessels. “RIB” refers to a “rigid inflatable boat” also known as “RHIB” or “rigid hull inflatable boat”.
Conventionally, a worker is required to step from the transfer vessel onto a flexible or rigid ladder or onto a rigid gangway at the side of the fixed structure. This is hazardous, particularly for those who are not accustomed to working in a marine environment. In all but the calmest of seas, relative motion between the transfer vessel and the fixed structure is substantial. Stepping from a moving transfer vessel onto a stationary ladder is difficult and it is easy to slip and, potentially, to fall into the water. This carries the danger of being crushed between the transfer vessel and the fixed structure. Because of the motion of waves and the effect of currents it is, in any case, difficult to keep the transfer vessel on station, i.e. in the correct position with respect to the fixed structure, and there is also the danger of collision between the transfer vessel and the fixed structure.
Transfer between two vessels, even those of generally comparable or similar size, can be equally perilous in view of the relative motion of the vessels with respect to one another.
For these reasons, safety regulations limit the transfer of personnel from a transfer vessel to a fixed structure to times when the sea conditions are within certain parameters, typically to circumstances where the wave height is less than about 0.7 m to 1 m. The consequence of this is that many working days are lost when wave conditions are such that access to the fixed structure is not possible. This can represent a major expense for those involved in the construction and maintenance of offshore facilities.